1. Field of the Invention
The present invention relates to investigation of earth formations. More particularly, the present invention relates to using resistivity measurements obtained during a plurality of passes of a resistivity tool in a borehole in order to obtain fractional flow characteristics of an earth formation. For purposes herein in both the specification and claims, the term “resistivity” is to be understood in its broadest sense to encompass the inverse thereof which is known as “conductivity”.
2. State of the Art
U.S. Pat. No. 5,497,321 describes methods of providing a log of fractional flow characteristics of formations surrounding an earth borehole. According to the methods, a logging tool is suspended in a borehole and resistivity measurements with different radial depths of investigation (e.g., ten inches, twenty inches, thirty inches, sixty inches and ninety inches into the formation) are made while the logging tool is moved through the borehole, thereby obtaining resistivity logs at different radial depths. Using a formation conductivity model, simulated resistivity log values are generated from estimated values of fractional flow parameters. The simulated resistivity log values are compared with measured values from the logging tool in order to obtain and error indication. The values for the fractional flow parameters are modified accordingly. An iterative procedure is used to find best fit values, from which an output of formation fractional flow characteristics is produced. An output of the filtrate loss is also produced.
The methods of U.S. Pat. No. 5,497,321 have proven very useful in providing good answer products. However, because the resistivity measurements utilized in the methods are from a single pass of a borehole tool which is usually run after drilling is complete, the answer products can be influenced by mud filtrate which has entered into the formation. As will be appreciated by those skilled in the art, because the salinity of the mud filtrate can differ significantly from the salinity of the formation fluids, and because a water-based mud-filtrate changes the water saturation in an oil zone around the borehole, the resistivity measurements made in an invaded formation can be affected.
In order to minimize the effects of invasion on measurements, resistivity logging-while-drilling (LWD) tools have been introduced. One such tool is the Compensated Dual Resistivity (CDR—a trademark of Schlumberger) tool which provides a borehole compensated resistivity with two depths of investigation. Another such tool is the Array Resistivity Compensated (ARC5—a trademark of Schlumberger) tool, which is an array resistivity tool. The ARC5 tool is an LWD tool which transmits electromagnetic waves which induce an eddy current in the formation and measures signals from the formation. These tools operate at about 2 MHz which is a higher frequency than a typical wireline tool. The ARC5 has three transmitters above and two transmitters below the receiver array. It measures five attenuation and five phase-shift values. Borehole compensation is achieved by using a linear mix of three different transmitters; the choice of the three depending on the transmitter spacing for which the correction is desired. Resistivity measurements are generated for depths of investigation into the formation of 10, 16, 22, 28, and 34 inches, and may be plotted as logs. LWD tools such as the CDR and ARC5 have the advantage over conventional resistivity measurement tools in that the resistivity measurement is made before significant filtrate invasion has taken place.